Mild steel blade... In an arena with hardox and titanium armors. Will be a short thick chuncky one that will degrade rapidly.

Bike axle. The most sturdy ones I know of are M14 treaded rods with add on inner races for the 3 piece ballbearings. Rather long too. I fear that will sheer off. And 6000 rpm on the blade. I believe the bearings of a bikewheel ain't made for that kind of speeds.
Because if a26" bikewheel runs 6000 rpm... 750 km/h